As laser technology has advanced to in-
clude higher laser powers, shorter puls-
es, and exotic wavelengths, the optical
components that support these lasers,
including cavity optics and beam-trans-
port components, have been forced to
evolve as well. With lasers operating
from the femtosecond regime to con-
tinuous-wave (CW), optics manufacturers are now being asked to provide a
multitude of solutions for a broad base
of customers.

New evaporative processes and novel process control have helped advance
the state of the art for optical coatings.
However, one of the limiting mecha-
nisms in the development of high-
er-power laser sources is the ability of
these coatings to withstand power with-
out exhibiting laser-induced damage or
degradation.

Integral to process development and
manufacturing is the ability to perform a well-defined and statistically
relevant measurement on the damage
threshold of optical components over a
broad range of wavelengths and pulse
formats. There is no one test that provides a panacea, so proper testing requires a multitude of lasers emulating
the wavelengths, pulse formats, and
pulse energies that the components
will see in their eventual applications.

There is, however, a general frameworkthat should be implement-ed in designing and imple-menting a specific damagetest procedure.

This article will discuss
one of the most important concepts that
must be understood to define a prop-
er test procedure and to interpret its
results. While a significant portion of
this measurement technique has been
embedded in the ISO-21254 laser-damage testing specification, we will discuss
a novel and exciting test method that
has furthered the ability to perform a
relevant damage test, and is in the early stages of implementation towards a
revised measurement standard.

As part of this discussion, we will be
talking strictly about laser damage occurring in thin films. This technique,
however, can be relevant for testing bulk
optical properties of materials as well.

The defect-drivendamage model

To understand certain mechanisms of
laser damage and to help design a proper test that interrogates the film surface,
one must make an assumption concerning the deposition process.

The basis behind the defect model is
that any film deposited on a substrate
will have defects that are intrinsic to
the specific deposition process and the
materials used in that process. 1 It is im-
portant to understand that these defects
can act as preferential damage precur-
sors because of absorption, electric field
perturbation, and in some cases micro-
focusing of the laser beam (see Fig. 1). 2

Sometimes, these defects are easily
observed using differential-interference
contrast (DIC) microscopy or dark-field
microscopy, or by imaging the scatter
from the optical surface. In other cases,
these defects cannot be observed with
optical instrumentation because of their
diminutive size or location within the
film. These defects tend to be randomly

A defect-driven damage model,
along with a raster-scan testing
approach, expand the scope of laser-induced damage testing (LIDT).